This invention relates to a capacitor and more particularly, but not exclusively, to a bypass capacitor for use in an inductive output tube (IOT).
IOTs are electron beam tube devices used, for example, to amplify high frequency signals for TV broadcasting and require voltages in the region of some tens of kilovolts for this operation.
According to the present invention, there is provided a capacitor comprising an electrically conductive tube, a layer of insulating material on the outer surface of the tube and electrically conductive material on the insulating material.
The geometry of a capacitor in accordance with the invention leads to a device having an inherently low inductance and also provides a continuous, distributed capacitance, making the capacitor particularly suitable for use as a bypass capacitor in high frequency electron beam tubes such as IOT amplifiers. This gives an improvement, for example, in sideband performance when used in digital television applications.
In a particularly advantageous embodiment of the invention, the tube provides support for the layer and the conductive material. As the tube acts as a substrate, neither the layer nor conductive material need be self-supporting. Advantageously, the insulating material has a greater length in the longitudinal axial direction of the tube than the electrically conductive material. Thus, the electrical path length between the tube and the conductive material may be made relatively long, providing good voltage hold-off.
Preferably, the insulating material covers substantially the whole of the outer surface of the tube. It may be applied for example by dip coating or in any other suitable way for the materials involved. One insulating material of particular suitability for the capacitor is Kapton (Trade Name) but other high dielectric constant insulator materials may be used. The Kapton may be applied as one or more layers on the outer surface of the tube or may be dip coated. The conductive material may be laid down on the insulating material by spraying, for example, or in another arrangement may be a metal foil wrapped around it.
Preferably the tube, insulating material and conductive material are arranged in a circularly symmetrical geometry about the longitudinal axis of the tube to minimize inductance. In one preferred arrangement, the length of the tube is at least twice its diameter.
The tube, being hollow, may if necessary surround electrical leads or other components of a device.
Advantageously, an electrical connector is included which is connected to the conductive material at substantially the mid-point of the axial extent of the conductive material. The connector may be substantially annular, making a connection with the conductive material around the inner periphery of the connector, again leading to low inductance. The connector may be an annular member arranged extensive in a plane normal to the longitudinal axis. In one embodiment, the connector includes apertures therein by which, for example, cooling air may be directed to other components of a device to which it is connected. The connector may include a mesh. The mesh may form only a part of the connector to provide a fluid path or it may make up a more substantial amount of the connector. For example, the connector may consist of a mesh arranged about a frame. The apertured or mesh configuration of the connector also give a reduction in weight which may be desirable in some uses.
In one embodiment the connector is electrically connected to the conductive material via a plurality of spring fingers. Similarly, the connector may also have a plurality of spring fingers around its outer periphery by which it makes electrical connection with a surrounding conductive member. The fingers provide good electrical connection but also permit the position of the connector to be adjusted readily. The connector may be supported by a separate electrically insulating support on which it is mounted.
A capacitor in accordance with the invention is particularly suitable for use where it is required to hold off high voltages, on the order of several tens of kilovolts. In one typical arrangement, the tube of the capacitor is maintained at 35 kV and the conductive material at ground potential during operation of a device to which it is connected.
According to a feature of the invention, an electron beam tube comprises an electron gun having a cathode and a capacitor in accordance with the invention connected as a bypass capacitor to short circuit stray and leakage high frequency power. The tube of the capacitor is preferably connected to cathode potential. Where the electron beam tube is combined with an annular input resonant cavity as in an IOT amplifier, the capacitor may conveniently be located in the volume surrounded by the inner wall of the cavity and substantially parallel thereto. The conductive material of the capacitor may be electrically connected to the inner wall.